A surface is generally defined as the external portion or limit of a body; the surface is often regarded as an interface between the solid body and its environment whether it is notably solid, liquid or gaseous.
When a drop of a given liquid is deposited on the surface of a solid, it adopts an equilibrium configuration and spreads over the surface to a varying extent. The angle θ, or contact angle, which is defined as the angle measured between the surface of the solid and the tangent to the drop, results from the equilibrium of the tensions of the three interfaces solid/liquid, solid/vapor and liquid/vapor. These quantities are related to one another by Young's equation. Typically, a set of measurements is performed on one and the same surface to determine a mean value of θ. Four cases can be distinguished, depending on the value obtained:                the liquid spreads spontaneously and wetting is said to be “perfect” (θ=0),        wetting is regarded as “good” (0<θ<90°),        wetting is said to be “poor” (90°<θ<180°),        no wetting occurs (θ=180°).        
The behaviors associated with the observations made at the macroscopic scale during measurements of contact angle may be different from those observed at smaller scales for which surface tensions of liquids play an important role. However, these behaviors do not detract from the value of the measurements performed on the macroscopic scale, as they make it possible to characterize the surfaces.
The characterization and investigation of the properties and of the behavior of the surfaces are abundantly documented in the literature, to which a person skilled in the art can refer. In this connection, we may notably mention the article by P. G. de Gennes, 1985 (Rev. Mod. Phys., vol. 57, pages 827-863).
The wettability of a surface can be altered by impregnation with a compound that penetrates more or less deeply into the material of which the structure is composed. This type of treatment requires the existence of affinity between the surface treated and the impregnating compound. However, the surface obtained is rarely homogeneous. Moreover, as the impregnating compound remains labile, the treatment must be repeated regularly to ensure its durability. The application of wax on wood corresponds to this type of treatment.
The application of a coating also leads to modification of the surface properties. Generally this type of treatment is applied to reduce the wettability of the surface with respect to water and increase the contact angle. The coating typically corresponds to a resin. The basic products used can be epoxy resins, polyurethanes, polyesters, or vinyl resins, associated with specific properties. The application of these compounds does not lead to the formation of strong bonds at the interface of the surface and the coating, which thus reduces the service life of this type of coating, depending on the environment. Moreover, they are generally films having a considerable thickness, notably greater than a micron, especially when the coating is applied to large areas of the order of several m2. At this thickness, there is a difference in optical properties between the untreated material and the material covered with the coating.
Glass is a material for which surface treatments are used extensively. At present, the surface tension of glass is only controlled by grafting alkyl siloxanes, of which there is a wide choice. However, the problem with this type of grafting is the stability of the bond between the glass and the silane (—Si—O—Si— bond), which soon undergoes hydrolysis, notably in a humid environment. This bond is fragile, depending on the environment, and especially in a basic environment.
There is therefore a real need to provide a durable treatment for modifying and/or controlling the surface tension of a material, applicable to any material and not altering the optical properties of said material thus treated.